In production of plate glass by float process, a float bath on which molten glass is discharged from a glass melting furnace and formed into a ribbon, comprises a metal casing lined with a refractory brick, and is filled with molten tin. Glass molten in the melting furnace is discharged on the surface of the tin bath, flows on the tin bath and advances to be formed into smooth plate glass. As usual plate glass (soda lime glass) for buildings, automobiles, etc., glass containing about 15 mass % of Na2O is used. Such glass is in contact with the molten tin at its bottom surface, and at the interface, Na2O in the glass diffuses into the tin bath. As a bottom refractory brick for such a float bath, Chamotte brick containing Al2O3 and SiO2 as the main components and having a mineral phase mainly comprising Mullite and Cristobalite is employed in view of characteristics, usefulness, cost, etc.
Na2O contained in the glass infiltrates from the surface of the float bath bottom refractory brick into its inside via the tin bath to form Nepheline, thereby to form a metamorphic phase different from the matrix of the refractory brick. The metamorphic phase increases in thickness with time in a direction perpendicular to the surface of the tin bath. Since Nepheline has a thermal expansion coefficient about three times as high as that of Mullite, peeling of the metamorphic phase from the matrix due to volume expansion of the metamorphic phase or due to thermal stress accompanying the change in temperature conditions of the float bath, i.e. so called flaking phenomena occur. The flakes, which have a specific gravity smaller than that of the molten tin, float up in the tin bath and scar the glass or generate foreign matters, and they can be a major cause of inhibiting smooth production of plate glass.
Heretofore, as a float bath bottom refractory brick which inhibits such flaking phenomena, one having a total alkali metal content up to 3% to a clay portion (Patent Document 1) and one employing a silicate material containing an alkali oxide having a particle diameter of at most 0.09 mm in an amount of from 1 to 3 mass % (Patent Document 2) have been disclosed. However, specifically, they contain two alkali oxides i.e. from 0.1 to 0.4% of Na2O and from 0.8 to 1.2% of K2O, and they will not contain 2% or more of K2O. Further, one employing a material having a particle diameter of at most 90 μm, which has a total content of Na2O and K2O of at most 1 mass % (Patent Document 3) and the like have been known. However, no document discloses effects of K2O as disclosed in the present invention.
Patent Document 1: JP-A-6-122543
Patent Document 2: JP-A-6-340471
Patent Document 3: JP-A-2003-277134